Spatial light modulators based on electromechanical grating devices are important for a wide range of applications, including display, data storage, spectroscopy and printing. Such systems require large numbers of individually addressable devices in either a linear or area array, with over a million addressable devices desirable for an area modulator array in a high-quality display.
Linear arrays are particularly advantaged over their area array counterparts by virtue of higher resolution, reduced cost, and simplified illumination optics. Of particular interest: linear arrays are more suitable modulators for laser light than are their two-dimensional counterparts. Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 (Bloom et al.) are one earlier type of linear array that offers a workable solution for high-brightness imaging using laser sources, for example. Another experimental type of linear array just recently disclosed and in early development stages is the flexible micromirror linear array, as described in the article “Flexible micromirror linear array for high resolution projection display” by Francis Picard, et al. in MOEMS Display and Imaging Systems, Proceedings of SPIE Vol. 4985 (2003). The prototype flexible micromirror linear array described in the Picard et al. article employs a line of reflective “microbridges” which are individually switched to modulate light to form a linear image.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued on Oct. 23, 2001, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.” The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device or, more simply, GEMS device, with GEMS standing for grating electromechanical system. The GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system and enables a simpler optical system design with smaller optical elements.
Display systems based on a linear array of GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,” issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,476,848, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SEGMENTED WAVEPLATE,” issued Nov. 5, 2002. Display systems based on GLV devices are disclosed in U.S. Pat. No. 5,982,553, entitled “DISPLAY DEVICE INCORPORATING ONE-DIMENSIONAL GRATING LIGHT-VALVE ARRAY” issued to Bloom et al. on Nov. 9, 1999.
Current GLV and GEMS color display system architectures employ three separate color paths, Red, Green, and Blue (RGB), each color path provided with a linear array of electromechanical grating devices. Each linear array of electromechanical grating devices modulates its component red, green, or blue laser light. The resulting modulated light beams are then combined onto the same output axis to provide a full-color image that is then scanned to the display screen. Referring to FIG. 1, there is shown a prior art display system 10 using three separate optical paths in this way.
For red color modulation, a red light source 70r, typically a laser, provides illumination that is conditioned through a spherical lens 72r and a cylindrical lens 74r and directed towards a turning mirror 82r. Light reflected from turning mirror 82r is modulated by diffraction at an electromechanical grating light modulator 85r. Modulated diffracted light from electromechanical grating light modulator 85r is diffracted past turning mirror 82r and to a color combiner 100, such as an X-cube or other dichroic combiner. The modulated light from color combiner 100 is then directed by a lens 75, through an optional cross-order filter 110 (not shown), to a scanning mirror 77 for projection onto a display surface 90. Green color modulation uses a similar set of components for providing light to color combiner 100, with a green light source 70g, typically a laser, providing illumination through a spherical lens 72g and a cylindrical lens 74g and directed towards a turning mirror 82g. Light reflected from turning mirror 82g is modulated by diffraction at an electromechanical grating light modulator 85g. Modulated diffracted light from electromechanical grating light modulator 85g is diffracted past turning mirror 82g and to color combiner 100. Similarly, blue light source 70b, typically a laser, provides illumination through a spherical lens 72b and a cylindrical lens 74b and directs light towards a turning mirror 82b. Light reflected from turning mirror 82b is modulated by diffraction at an electromechanical grating light modulator 85b, diffracted past turning mirror 82b and to color combiner 100.
In order to maintain precise color registration with the system of FIG. 1, each of the three linear arrays, electromechanical grating light modulators 85r, 85g, and 85b, must be aligned to each other, to within sub-pixel tolerances. This precision of alignment is difficult to obtain. Moreover, this alignment must prove stable so that it can be maintained over a range of temperatures and other environmental conditions, such as shock and vibration. Color combiner 100 is a particularly costly component and can be the source of undesirable image aberrations.
One design solution that eliminates the color channel alignment problem is color-sequential operation, whereby a single electromechanical grating light modulator serves to modulate each color light in sequence. This type of system is described, for example, in the disclosure of U.S. Pat. No. 6,411,425 entitled “Electromechanical Grating Display System with Spatially Separated Light Beams” to Kowarz et al. (see cols. 9-10). However, as noted in the '425 Kowarz et al. disclosure, color-sequential techniques, by modulating only one color at a time, effectively waste two-thirds of the available light. Thus, although color-sequential solutions minimize alignment concerns, these solutions have performance drawbacks.
Thus, it can be seen that there would be advantages to a projection solution that provides the performance benefits of electromechanical grating light modulators without requiring frequent adjustment of alignment for each color path or requiring complex and costly mounting apparatus that would be able to maintain alignment over a wide range of operating conditions, and that provides a compact, low-cost apparatus, without the requirement for a color-combining prism.